Electric heater for a motor vehicle

ABSTRACT

An electric heater for a motor vehicle uses heat generated by power semiconductors as the heat source. The heat output by the power semiconductors is used directly for heating. The power semiconductors are regulated by circuit regulators to be able to adjust the heating power continuously. In addition, switching devices are provided which interrupt or shut down the respective branch circuits individually in the event of short circuits in the power semiconductors.

BACKGROUND INFORMATION

[0001] The present invention relates to an electric heater for a motorvehicle, using the heat generated by power semiconductors as the heatsource.

[0002] Such a heater is known from German Patent No. 34 42 350 C2. Withthis known heater, the power semiconductor controls the electric drivemotor. The power semiconductor is connected to a cooling body throughwhich a liquid coolant flows, so the heat generated is transferred tothe liquid coolant by heat exchange. The liquid coolant circulates in aclosed line system having a pump and the actual heater installation.

[0003] The efficiency of this known electric heater is not especiallygreat, because the heat generated by the power semiconductor must beconverted repeatedly. In addition, the heater installation has acomplicated design, depends on the engine current present and thuscannot be regulated independently of the latter.

SUMMARY OF THE INVENTION

[0004] The object of the present invention is to create an electricheater of the type defined in the preamble where the efficiency isgreatly increased with a simple design and independent regulation ofheating power is possible.

[0005] This object is achieved according to a first embodiment of thepresent invention by connecting several branch circuits, each with onepower semiconductor operated in high power loss operation, in parallelfor generation of heat, or according to a second embodiment byconnecting several branch circuits, each with two series-connected powersemiconductors operated in high power loss operation, in parallel forgeneration of heat.

[0006] In these embodiments, the current is converted directly into heatby the power semiconductors, which greatly increases efficiency. Anotheradvantage of the new heater is that no additional control module isneeded for the heater. Installation of the heater in the motor vehicleis also greatly simplified. In addition, the cabling complexity andmanufacturing costs of the new electric heater are also reduced.

[0007] No separate fuse protection for the heater in the vehicleelectrical system is necessary. When starting operation of the heater,the high starting current surge can be prevented by a regulated smoothcurrent rise. The new heating module can be cascaded in any desiredfashion to increase the heating power and can also be integrated easilyinto a fan regulator.

[0008] To protect the power semiconductors, one embodiment provides fora switching device that responds to overload to be connected in serieswith the power semiconductor in each branch circuit. In the event of afault, the branch circuit affected can be shut down with this switchingdevice without having to lose heater function as a whole. Heating poweris reduced only by the ratio of defective branch circuits to totalbranch circuits.

[0009] According to one embodiment, regulation of the heating power iseasily made possible by the fact that the power output by the powersemiconductors can be regulated individually by a common predeterminedsetpoint and by actual values derived from the power semiconductors, orby the fact that the powers output by the respective first powersemiconductors of the branch circuits can be regulated individually by acommon predetermined setpoint and by actual values derived at thesepower semiconductors, and the powers output by the respective secondpower semiconductors can be regulated individually by a fixedpredetermined control voltage and by actual values derived at thesepower semiconductors.

[0010] If the branch circuits are to supply power at the output to alow-resistance series resistor as a load impedance, then the heatgenerated by the series resistor can contribute to an increase inheating power. Each power semiconductor can supply power to anindividual series resistor. All the power semiconductors may also supplypower to a common series resistor, or groups of power semiconductors mayeach be connected to a group-individual series resistor.

[0011] The switching devices for interrupting the branch circuits can beimplemented in various ways. Thus, according to one embodiment, theswitching devices may be designed as a printed conductor part of thebranch circuits which burn out in the event of a fault at the elevatedcurrent occurring in the respective branch circuit. The same effect canalso be achieved by looping the switching devices as shunts into thebranch circuits, which burn out in the event of a fault at the elevatedcurrent occurring in the respective branch circuit, in which case theshunt can also be used to derive another control signal. Finally, thebranch circuit can also be interrupted by using the connecting wires ofthe power semiconductors which burn out in the event of a fault at theelevated current occurring in the respective branch circuit.

[0012] A controlled reduction or interruption in the current in adefective branch circuit occurs when measures are taken to ensure thatin the event of a short circuit in one of the two power semiconductorsconnected in series in a branch circuit, an additional control signalcan be derived from the defective branch circuit to reduce the poweroutput by the respective second power semiconductor or switching it to adisconnect status. The control signal picked off at the shunt can beused as the control signal.

[0013] The structural design of the new electric heater can besimplified by designing it as a heater module, with the powersemiconductors mounted in thermal contact on a cooling body, with theheat transfer via the cooling body being improved.

[0014] Simple temperature monitoring can be achieved with the electricheater by the fact that the power semiconductors and/or the cooling bodyare monitored by temperature sensors to detect whether a predeterminedmaximum temperature is exceeded, and by the fact that the output signalsof the temperature sensor(s) reduce the power output by the respectivepower semiconductors or all the power semiconductors or switch them to adisconnect status. If the power semiconductors are monitored byindividual temperature sensors, the expense of this is reduced byintegrating the temperature sensors into the power semiconductors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention will now be explained in greater detail onthe basis of embodiments illustrated in the drawings, which show:

[0016]FIG. 1: a circuit diagram of an electric heater with n parallelbranch circuits, each containing one power semiconductor, and

[0017]FIG. 2: a circuit diagram of an electric heater with n parallelbranch circuits, each having two series-connected power semiconductors.

DETAILED DESCRIPTION

[0018] As FIG. 1 shows, branch circuits Z1, Z2, . . . Zn are connectedin parallel to one another at power supply voltage Ubatt, with eachbranch circuit Z1, Z2, . . . , Zn having a power semiconductor FET1,FET2, . . . , FETn. The connection to power supply voltage Ubatt is byway of switching devices FUSE1, FUSE2, . . . , FUSEn, which perform anindividual interruption of branch circuit Z1, Z2, . . . , Zn in theevent of a fault, e.g., a short circuit of the power semiconductor atwhich the multiple current occurs. A shunt which is not shown in detailmay be added to the connection of power semiconductors FET1, FET2, . . ., FETn at the ground potential, where an individual actual value Ist1,Ist2, . . . Istn can be derived for the branch circuit Z1, Z2, . . . ,Zn. In addition to the actual value picked off at the shunt, a setpointIsoll is supplied to the gate terminal of power semiconductors FET1,FET2, FETn via a comparator or operational amplifier that serves as acircuit regulator to permit continuous regulation of the power in therespective power semiconductor. If the actual value exceeds setpointIsoll, then the circuit regulator switches the power semiconductor intothe disconnect status or reduces the power output. In addition,switching device FUSE1, FUSE2, . . . , FUSEn can completely interruptbranch circuit Z1, Z2, . . . , Zn in the event of a short circuit ofrespective power semiconductor FET1, FET2, . . . , FETn.

[0019] Printed conductor segments of branch circuit Z1, Z2, . . . , Znthemselves can be used as switching devices FUSE1, FUSE2, FUSEn.Depending on the design of branch circuits Z1, Z2, Zn and the respectivecircuit regulator, the current may increase to a level 25 to 50 timeshigher in the event of a short circuit, so the printed conductor partburns out. The shunt can also be used as a switching device if it burnsout with this current rise and interrupts branch circuit Z1, Z2, . . . ,Zn. Even the connecting wires of power semiconductors FET1, FET2, . . ., FETn can be dimensioned to assume the function of switching devicesFUSE1, FUSE2, . . . , FUSEn. The electric heater of this type may bedesigned as a heater module, mounted on a cooling body and integratedinto a fan regulator; furthermore, the heater module itself need nolonger be fused with respect to the vehicle's electrical system.However, it may be necessary to fuse the feeder lines to the heatermodule.

[0020] In the embodiment according to FIG. 2, each branch circuit Z1,Z2, . . . , Zn has two series-connected power semiconductors FET11 andFET12, FET21 and FET22, . . . FETn1 and FETn2, each controlled by itsown circuit regulator. As in the embodiment according to FIG. 1, aswitching device FUSE1, FUSE2, FUSEn and a shunt can be looped intobranch circuits Z1, Z2, . . . , Zn. Switching devices FUSE1, FUSE2, . .. , FUSEn in turn can be designed in the variants described. Controlsignals characterizing actual value Ist11, Ist21, . . . Istn1 of branchcircuit Z1, Z2, . . . , Zn can be picked off at the shunts of the branchcircuits and sent to the circuit regulators of the respective firstpower semiconductors FET11, FET21, . . . , FETn1 to which can also besent a setpoint Isoll to regulate the power in branch circuit Z1, Z2, .. . , Zn. The second power semiconductors FET12, FET22, . . . , FETn2are controlled by separate circuit regulators to which are sent a fixedpredetermined control voltage ust and an actual value Ist12, Ist22, . .. , Istn2, which is derived from the voltage drop at the first upstreampower semiconductor FET11, FET21, FETn1. In the event of a short circuitor defect in a power semiconductor such as FET11 with this design of thecircuit regulators, the respective second power semiconductor, such asFET12, in branch circuit Z1 can be shut down or the power output by itcan be reduced. However, the functioning of the remaining system is notaffected, and the heating power is merely reduced by the ratio ofdefective branch circuits to total branch circuits.

[0021] If both power semiconductors, e.g., FET21 and FET22, areshort-circuited, then the switching device, e.g., FUSE2 as in theembodiment according to FIG. 1, goes into operation and interrupts thebranch circuit, e.g., Z2, at the high current rise occurring.

[0022] If only one common heat-dissipating, low-resistance seriesresistor is used as the load impedance for all branch circuits Z1, Z2, .. . , Zn to increase the heating power, then this resistor is loopedinto the common line leading to battery voltage Ubatt. This seriesresistor does not change the operation of the electric heater, it merelylimits the current rise to a lower level in the event of a short circuitin a single power semiconductor (FIG. 1) or both power semiconductors(FIG. 2), but this lower level is still sufficient for a reliableresponse of switching device FUSE1, FUSE2, . . . , FUSEn. The heatgenerated by the series resistor is also used for heating, but itentails a power distribution which can be utilized at a predeterminedmaximum heating power to expand the temperature use range for theheater.

[0023] Each power semiconductor or each pair of power semiconductors canalso be connected to battery voltage Ubatt across an individual seriesresistor. Groups of branch circuits may also supply a series resistor.In any case, all the series resistors are involved in the production ofheat.

[0024] Temperature monitoring can easily be incorporated into the newheater. Thus, a temperature sensor may be provided for each powersemiconductor and may also be integrated into the power semiconductor.If a predetermined maximum temperature is exceeded at the powersemiconductor, the output signal of the temperature sensor then controlsthe respective power semiconductor so that its power output is reducedor it is completely shut down.

[0025] It is also possible to provide just one temperature sensor formeasuring the temperature of the cooling body, with all the powersemiconductors of the electric heater being in thermal contact with it.If the temperature of the cooling body exceeds a predetermined maximumtemperature, then all the power semiconductors are controlled with theoutput signal of the temperature sensor in such a way that their poweroutput is reduced or they are completely shut down. Different values ofthe output signal of the temperature sensor can be used for thispurpose, with the output signal initially triggering a power reductionat the first lower value and a complete shutdown at the second highervalue of the output signal.

1. An electric heater for a motor vehicle, using heat generated by powersemiconductors as the heat source, characterized in that several branchcircuits (Z1, Z2, Zn), each with one power semiconductor (FET1, FET2,FETn) operated in high power loss operation, are connected in parallelfor generation of heat.
 2. An electric heater for a motor vehicle, usingheat generated by power semiconductors as the heat source, characterizedin that several branch circuits (Z1, Z2, Zn), each with twoseries-connected power semiconductors (FET11, FET12; FET21, FET22;FETn1, FETn2) operated in high power loss operation, are connected inparallel for generation of heat.
 3. The electric heater according toclaim 1 or 2 , characterized in that a switching device (FUSE1, FUSE2,FUSEn) which responds to overload is connected in series to the powersemiconductor (FET1, FET2, FETn) in each branch circuit (Z1, Z2, Zn). 4.The electric heater according to claim 1 or 3 , characterized in thatthe power output by the power semiconductors (FET1, FET2, FETn) can beregulated individually by a common predetermined setpoint (Isoll) and byactual values (Ist1, Ist2, . . . , Istn) derived from the powersemiconductors (FET1, FET2, . . . , FETn).
 5. The electric heateraccording to claim 2 or 3 , characterized in that the powers output bythe respective first power semiconductors (FET11, FET21, FETn1) of thebranch circuits (Z1, Z2, Zn) can be regulated individually by a commonpredetermined setpoint,(Isoll) and by actual values (Ist11, Ist21, . . ., Istn1) derived at these power semiconductors, and the powers output bythe respective second power semiconductors (FET12, FET22, . . . , FETn2)can be regulated individually by a fixed predetermined control voltage(ust) and by actual values (Ist12, Ist2, . . . , Istn2) derived at thesepower semiconductors.
 6. The electric heater according to one of claims1 through 5, characterized in that the power semiconductors (FET1, FET2,. . . , FETn; FET11, FET12, FET21, FET22, . . . FETn1, FETn2) areoperated in the short circuit.
 7. The electric heater according to oneof claims 1 through 6, characterized in that the branch circuits (Z1,Z2, . . . , Zn) supply power at the output to a low-resistance seriesresistor as a load impedance.
 8. The electric heater according to one ofclaims 1 through 7, characterized in that the switching devices (FUSE1,FUSE2, . . . , FUSEn) are designed as a printed conductor part of thebranch circuits (Z1, Z2, . . . , Zn) which burn out in the event of afault at the elevated current occurring in the respective branch circuit(Z1, Z2, . . . , Zn).
 9. The electric heater according to one of claims1 through 7, characterized in that the switching devices (FUSE1, FUSE2,FUSEn) are looped as shunts into branch circuits (Z1, Z2, Zn) which burnout in the event of a fault at the elevated current occurring in therespective branch circuit (Z1, Z2, . . . , Zn).
 10. The electric heateraccording to one of claims 1 through 7, characterized in that theconnecting wires of the power semiconductors (FET1, FET2, FETn; FET11,FET12, FET21, FET22, FETn1, FETn2) are used as switching devices (FUSE1,FUSE2, FUSEn) which burn out in the event of a fault at the elevatedcurrent occurring in the respective branch circuit (Z1, Z2, Zn).
 11. Theelectric heater according to one of claims 2, 3 and 5 through 7,characterized in that in the event of a short circuit in one of the twopower semiconductors (FET11 or FET12; FET21 or FET22; FETn1 or FETn2)connected in series in a branch circuit (Z1, Z2, Zn), an additionalcontrol signal can be derived from the defective branch circuit (Z1, Z2,Zn) to reduce the power output by the respective second powersemiconductor (FET12 or FET11; FET22 or FET21; FETn2 or FETn1) or switchit to a disconnect status.
 12. The electric heater according to one ofclaims 1 through 11, characterized in that it is designed as a heatermodule, with the power semiconductors (FET1, FET2, . . . , FETn; FET11,FET12; FET21, FET22; FETn1, FETn2) mounted in thermal contact on acooling body.
 13. The electric heater according to claim 12 ,characterized in that the power semiconductors (FET1, FET2, . . . ,FETn; FET11, FET12, FET21, FET22, . . . , FETn1, FETn2) and/or thecooling body are monitored by temperature sensors to detect whether apredetermined maximum temperature is exceeded, and in that the outputsignals of the temperature sensor(s) reduce the power output of therespective power semiconductors or all the power semiconductors orswitch them to a disconnect status.
 14. The electric heater according toclaim 13 , characterized in that the temperature sensors are integratedinto the power semiconductors (FET1, FET2, . . . , FETn; FET11, FET12,FET21, FET22, . . . , FETn1, FETn2).